(1) Field of the Invention
The present invention relates to current drivers and particularly to current drivers usable as apparatus such as display drivers for organic EL (electro luminescence) panels and the like.
(2) Description of Related Art
FIG. 22 illustrates an overall configuration of a conventional current driver 30. The conventional current driver 30 uses a reference current Iref from a reference current source REF (or a current driver at the previous stage) as an input and includes: a setting reference transistor T3001L; a supply reference transistor T3001R; a bias voltage generating transistor T3003; and driving transistors T3004a and T3004b. 
The setting reference transistor T3001L is provided between a power supply node Vdd and the reference current source REF and has its gate and drain connected to each other. The supply reference transistor T3001R is provided between a power supply node Vdd and the bias voltage generating transistor T3003 and has its gate connected to the gate of the setting reference transistor T3001L. The bias voltage generating transistor T3003 is provided between the supply reference transistor T3001R and a ground node and has its gate connected to the gates of the driving transistors T3004a and T3004b. Each of the driving transistors T3004a and T3004b is provided between a display element circuit (not shown) and a ground node.
Now, it will be described how the current driver 30 illustrated in FIG. 22 operates. In the conventional current driver 30, a current mirror formed by the setting reference transistor T3001L, the supply reference transistor T3001R and the bias voltage generating transistor T3003 generates a bias voltage Vbias from the reference current Iref. Each of the driving transistors T3004a and T3004b receives, at its gate, the bias voltage Vbias generated by the current mirror and generates an output current Iout. In this manner, output currents Iout are caused to flow uniformly, thereby current-driving a plurality of display element circuits in a display panel.
In the conventional current driver, however, in order to enhance the accuracy (current mirror accuracy) of the ability of generating an output current Iout having a current value equal to or proportional to the current value of the reference current Iref, the current value of the reference current Iref input to the current mirror needs to be increased. With the foregoing method, electric power cannot be used effectively, and heavy loads are placed on the transistors T3001R and T3001L forming the current mirror. In addition, in the conventional current driver, a bias voltage (Vbias) varies because of transistor capacitive coupling, so that the current values of output currents Iout can vary.